Shimano Saint Der on Horizontal Dropouts?

I bought my sister a Bianchi SASS last year because she was looking for a bike and it was a steal. Problem is there is only one gear and she is not into that sort of thing. So I've got to methods to rig it up with gears

1. Use a Sram iMotion 9 w/ disc brakes. (this option is pretty expensive)
2. Use a Shimano Saint derailur with a Saint hub. (cheaper option and I already own some of the parts)

I realize that I'll have to create my own cable routings for either options and I'm not worried about that. Does anyone have any advice/experience with this issue?